Three figures of speech related to grass in "Song of Myself" are as follows:
It must be the flag of my disposition, out of hopeful green stuff woven.
In the quote above, Whitman uses a metaphor when he says grass is the "flag of my disposition." A metaphor compares two unlike things without using the words like or as. Here the waving green grass seems to the speaker like a green flag waving in the wind. For Whitman, this is a positive image: the grass is like his disposition because they are both hopeful. The fact that the grass is hopeful also shows that Whitman personifies or assigns human characteristics to it.
I guess the grass is itself a child, the produced babe of the vegetation.
The second quote is also a metaphor that personifies the grass. In this case, the speaker likens the grass to a child. Both are alike because they start out as "babes" that grow. As he would a child, the speaker will "tenderly use" the curling grass.
I believe a leaf of grass is no less than the journey-work of the stars.
In the third quote, the speaker suggests the cosmic unity of all creation by using a metaphor to compare a blade of grass to the work done by the stars. This could be seen, too, as an oxymoron, or juxtaposition of opposites, because a leaf of grass is tiny and insignificant while stars are huge and magnificent lights in the heavens. It could also be seen as hyperbole or exaggeration to think of a blade of grass as the work of stars, because, again, grass is so insignificant. Whitman, however, is trying to suggest that it is not, in fact, insignificant but a part of the sacred, universal whole. I might also suggest that Whitman explodes normal categories of figurative speech: all of his poetry is exuberantly hyperbolic, for example, and in his strong identification with all of nature and the universe, he tends to personify everything.